The membrane separation process has characteristics of energy saving, space saving, an improvement in filtrated-water quality, and others, so that the use thereof has been increasing in various fields. For example, a microfiltration membrane or an ultrafiltration membrane is applied to a water-purifying process for producing industrial water or tap water from river water, ground water or treated waste water, or is applied to a pre-treatment in a seawater desalination reverse osmosis membrane treatment process.
When a raw liquid is filtrated through a membrane, the quantity of contaminants adhering to surfaces of the membrane or the inside of holes in the membrane increases in accordance with the liquid quantity filtrated through the membrane to cause a problem of lowering in the membrane filtrate quantity, or a rise in the membrane differential pressure.
Thus, the following cleaning has been put into practical use: air cleaning of introducing bubbles to the raw liquid side of membranes and swinging the membranes to bring the membranes into contact with each other, thereby scraping off contaminants adhering to the membranes; or physical cleaning of, for example, pushing a membrane filtrate or a clean liquid into a membrane along a direction reverse to a filtrating direction of the membrane to perform back-pressure washing for excluding contaminants adhering to surfaces of the membrane or the inside of holes in the membrane.
When the quantity of contaminants in a raw liquid is large, the above-mentioned cleaning method may not be sufficient to remove the contaminants adhering to the membrane surfaces. As a method for heightening the cleaning effect, Patent Documents 1 to 4 each describe a cleaning method of changing the liquid level at the raw liquid side of a membrane while bubbles are introduced thereinto from an air-diffusing device located inside a module of the membrane. Patent Document 5 suggests a cleaning method of raising and lowering the liquid level at the raw liquid side of a membrane while bubbles are introduced thereinto from an air-diffusing device located at the lower part of a module of the membrane. These methods make use of pulsatory motions of the liquid surface on the basis of waves generated when the bubbles break down in the liquid surface, thereby aiming to produce an advantageous effect of peeling off contaminants adhering to the membrane surface intensely.